An optical transmission system has to contend with a number of different optical fiber nonlinearities, one of which is commonly referred to as Raman tilt introduced by stimulated Raman scattering (SRS). Stimulated Raman Scattering (SRS) is an important non-linear effect in fibers that can be used to build Raman amplifiers, but that also degrades Wavelength Division Multiplexed (WDM) signals due to channel-to-channel cross talk and spectral distortion (gain tilt). The Raman tilt is defined (in dB) as the difference between the power (in dB) of the longest wavelength channel and the power (in dB) of the shortest wavelength channel. The Raman tilt, or effect, becomes particularly troublesome when an appreciable level of optical power distributed over a certain range of wavelengths is pumped into an optical fiber. In that instance, the Raman gain is tilted in favor of the channels having the longer wavelengths. The Raman effect attenuates the power levels of the optical channels of the lower wavelengths, but increases the power levels of the optical channels of the higher wavelengths by shifting the power from the former channels to the latter channels. The Raman tilt thus degrades the Signal-to-Noise Ratio (SNR) of the signals in the lower wavelength channels and thus seriously degrades their performance.
Typically multi-channel optical transmission systems use broadband optical repeaters to periodically amplify signals. To maximize the number of useful channels and reduce the effects of Raman tilt, a received signal at a repeater may be pre-tilted (i.e., in the opposite direction of the Raman tilt) so as to reduce the variation of Signal-to-Noise Ratio from channel to channel. In addition, when Raman pumps are used for producing gain, the spectrum of the optical channels may also acquire a Raman tilt that requires a repeater with a pre-tilt in the opposite direction. Typically in either case, a repeater with a tilted gain response is used and in general, the repeater is operated in constant gain mode.
However, during transient events (i.e., a sudden change in channel count), individual channel gains may vary by as much as half of the total correction tilt because a constant gain control is used.